1. Field of the Invention
This invention relates to an electro-optical device such as a liquid crystal display device, and more particularly to a display device of an active matrix.
2. Description of Prior Art
Recently an active matrix for driving a liquid crystal display has been studied intensively and used practically. In the conventional active matrix circuit, a capacitor which has liquid crystal in between pixel electrodes and counter electrodes is formed, and charges going into and out of this capacitor are controlled by a thin film transistor(TFT). For stable displaying, it was required to keep a voltage applied across both electrodes of this capacitor a constant value, but some factors made this difficult.
Most significant factor was that the charge leaked from the capacitor even while the TFT was in off state. As another factor, the charge leaking occurred also inside the capacitor, but the former leaking from the TFT was generally about 10 times as large as the latter leaking. When this leak was strong, brightness of an image was varied in the same frequency as a frame frequency, and this phenomena is called "flicker". The perturbation of the voltage (.DELTA.V) caused by capacitor coupling of a TFT gate signal and a pixel voltage due to a parasitic capacitance between them was also one of the factors.
To avoid these problems, an auxiliary capacitor (or an additive capacitor) connected in parallel to a pixel capacitor has been used. This situation is illustrated as a circuit diagram in FIG. 1(A). An equivalent circuit of FIG. 1(A) can be obtained by providing a voltage control means for controlling the voltage of an electrode of an auxiliary capacitor 100 so as to be substantially equal to a voltage of a counter electrode. Thus, a time constant of discharge of a pixel capacitance is increased by such an auxiliary capacitor. .DELTA.V is represented as follows: EQU .DELTA.V=C'V.sub.G /(C.sub.LC +C'+C)
where V.sub.G, C.sub.LC, C and C' represent a gate pulse (signal voltage), a pixel capacitance, an auxiliary capacitance and a parasitic capacitance between the gate electrode and the pixel electrode, respectively. If C is larger than C' or C.sub.LC, .DELTA.V can be reduced. A film of silicon oxide or silicon nitride formed by CVD method has been conventionally used as dielectric material for the auxiliary capacitor. PA1 a substrate 1; PA1 a wiring provided on said substrate; PA1 an anodic oxide film provided on said wiring and comprising an oxide of a material of said wiring; and PA1 a pixel electrode 17 provided on said substrate and provided in contact with said anodic oxide film. PA1 a substrate; PA1 a gate electrode of a transistor provided on said substrate; PA1 an anodic oxide film provided on said gate electrode and comprising an oxide of a material of said gate electrode; PA1 a wiring provided on said substrate; PA1 another anodic oxide film provided on said wiring and comprising an oxide of a material of said wiring; PA1 a pixel electrode provided on said another anodic oxide film and connected with one of source and drain of said transistor; PA1 a counter electrode provided on another substrate; and PA1 an electro-optical modulating layer provided between the substrates; and PA1 means for controlling a potential of said wiring to be substantially equal to a potential of said counter electrode. PA1 a substrate; PA1 a wiring provided on said substrate and functioning as one of a pair of electrodes of a capacitor; PA1 an anodic oxide film provided on said wiring and comprising an oxide of a material of said wiring and functioning as a dielectric of said capacitor; and PA1 a transparent conductive film provided on said anodic oxide film and functioning as a pixel electrode and the other one of said pair of electrodes.
In the film formed by CVD method, however,if the base layer of the film is rugged, a ruggedness (unevenness) on the layer has a tendency to be enhanced, and this enhancement of the unevenness raises the number of defects such as fine holes, etc. Especially to raise an aperture ratio of a display device, it is difficult to assign a large area to the auxiliary capacitor as described above. Therefore, it has been required to reduce thickness of the dielectric material or to utilize material having high dielectric constant. However, the dielectric constant of silicon oxide is small (about 4). On the other hand, the dielectric constant of silicon nitride is large (about 9), however, the film quality of silicon nitride is not good.